Communications
DOI: 10.1002/anie.200602914
Self-Assembly
Electrostatic Self-Assembly of Polystyrene Microspheres by Using
Chemically Directed Contact Electrification**
Logan S. McCarty, Adam Winkleman, and George M. Whitesides*
Herein we describe a process—based on contact electrification and electrostatic interactions—that directs the selfassembly of chemically modified polystyrene microspheres
to form three-dimensional microstructures. When two solid
surfaces are brought into contact and separated, charge is
often transferred from one surface to the other in a process
known as contact electrification.[1, 2] We can predictably and
rationally control the contact electrification of polystyrene
microspheres, and use the resulting charged materials for
electrostatic self-assembly. We control the contact electrification of these microspheres by introducing immobilized ions
and mobile counterions: the choice of these ions determines
the electrostatic charges that these beads acquire through
contact before and during the assembly process. Oppositely
charged microspheres assemble into uniform spherical microstructures under the influence of electrostatic forces. Sequential steps of self-assembly can create multilayered microstructures.
There are many examples of electrostatic self-assembly of
charged ions, polyelectrolytes, and colloids in solution.[3–8]
Xerography, an example of dry electrostatic self-assembly,
uses corona discharge from a high-voltage electrode to create
a charge on the imaging drum, and contact electrification to
create an opposite charge on the toner particles.[9] Contact
electrification can also direct the self-assembly of millimetersized spheres into ordered two-dimensional lattices.[10] (That
process used the inherent differences in contact electrification
of various polymers, in contrast to the rational, chemically
directed contact electrification we describe herein.) Patterns
of charge, created by electron-beam writing,[11] an atomic
force microscopy (AFM) tip,[12] or electrical microcontact
printing[13] on a dielectric surface, can guide the self-assembly
of micro- or nanoparticles with sub-100 nm lateral resolution.[14]
Although contact electrification is a familiar phenomenon, the detailed mechanisms of contact electrification are
not known, and it is likely that different mechanisms may be
involved depending on the specific materials and environmental conditions. There is one class of materials that exhibits
[*] L. S. McCarty, A. Winkleman, Prof. G. M. Whitesides
Department of Chemistry and Chemical Biology
Harvard University
12 Oxford Street, Cambridge, MA 02138 (USA)
Fax: (+ 1) 617-495-9857
E-mail: gwhitesides@gmwgroup.harvard.edu
[**] This research was supported by the Army Research Office (W911NF04-1-0170).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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predictable contact electrification and for which Diaz and coworkers have proposed a plausible mechanism of chargetransfer:[15–17] the contact charging of ionomers (polymers
with covalently bound ionic functional groups) is believed to
result from the transfer of mobile ions from the ionomer to
another material (Figure 1).
Figure 1. A schematic representation of the ion-transfer model of
contact electrification.
According to this model, an ionomer with covalently
bound cations transfers some of its mobile counterions to
another surface upon contact; this transfer results in a positive
charge on the ionomer, and a negative charge on the other
surface. Using an ionomer with covalently bound anions
would yield the opposite charges. Diaz and co-workers
compiled extensive experimental support for this mechanism,
including X-ray photoelectron spectroscopy (XPS) measurements of the contacted surface that demonstrate transfer of
the mobile ion but not the covalently bound ion.[18] Because
this mechanism for the contact electrification of ionomers is
reasonably well-understood, it is possible to rationally design
components to become charged through contact electrification, and use those components for electrostatic self-assembly.
We prepared poly(styrene-co-divinylbenzene) microspheres with covalently bound tetraalkylammonium functional groups (1) and covalently bound sulfonate functional
groups (2) as shown in Scheme 1. The azobenzene moiety in 2
imparted an orange color to those beads, while the other
beads (1) were colorless. We prepared beads with diameters
ranging from 5 to 200 mm with both types of functionality, and
characterized the insoluble cross-linked beads by elemental
analysis and IR spectroscopy. (The Supporting Information
provides experimental details.)
We investigated the contact electrification of each type of
bead using the following procedure: About 40, 200-mmdiameter beads were agitated in a 5-cm-diameter aluminum
dish for approximately 5 min. We measured the charges on
individual beads using a device described in the Supporting
Information; briefly, the device consisted of an aluminum
tube connected to an electrometer. As individual charged
beads were passed through the tube, the electrometer
recorded the charge induced on the exterior of the tube.
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Chemie
Scheme 1. Chemical modification of cross-linked polystyrene microspheres to yield spheres with tetraalkylammonium (1) and sulfonate
(2) functional groups. The Supporting Information provides experimental details. NMP = 1-methyl-2-pyrrolidone, DMF = dimethylformamide, P = polymer.
According to Gauss?s Law, this induced charge is the same as
the charge on the bead.
Figure 2 shows histograms of charge measurements of
200-mm beads with each type of functionality. As predicted by
the ion-transfer hypothesis, all of the beads with the
tetraalkylammonium functionality (1) were positively
charged, while all of the beads with the sulfonate functionality
(2) were negatively charged. Assuming that the charge is
uniformly distributed on the surface of each bead, the
magnitude of the charge (ca. 0.01 nC per bead) corresponds
to approximately one elementary charge per 2000 nm2. Since
the density of ionic functional groups on the surface of each
bead is probably on the order of one functional group per
10 nm2, only about 0.5 % of the mobile ions on the bead
surface are transferred during contact electrification.
Figure 2. Histograms (with normal distributions superimposed) of the
measurements of charges on individual 200-mm-diameter beads.
a) Tetraalkylammonium beads (1). b) Sulfonate beads (2). N = sample
size, st. dev. = standard deviation.
Angew. Chem. Int. Ed. 2007, 46, 206 –209
Figure 3 a shows a schematic representation of the process
of self-assembly. In a typical experiment, we combined
approximately 0.5 mg of dry, 200-mm-diameter sulfonatefunctionalized beads (2) with approximately 0.5 mg of dry, 20mm-diameter tetraalkylammonium-functionalized beads (1)
in an aluminum dish. We agitated the dish until the beads
were thoroughly mixed. Within seconds, each orange 200-mm
bead (2) became coated with a slightly disordered monolayer
of the colorless 20-mm beads (1), as shown in Figure 3 b.
Mixtures with the charges reversed (i.e., the larger sphere was
positively charged) or with different sizes of microspheres
yielded similar structures. We reproduced these results under
a wide range of ambient laboratory conditions (ca. 20–25 8C
and ca. 30–80 % relative humidity (RH)). The smallest
assemblies were achieved with a combination of 5-mm and
50-mm beads (shown in the Supporting Information as
Figure S1.) When the two oppositely charged spheres were
the same size, extended aggregates formed with local
Coulombic ordering (e.g., (+)( )(+)( )) but no long-range
order.
To achieve uniform coverage of the large spheres, we used
an excess of the small spheres (at least 10-times more than
would be needed for monolayer coverage). Thus, we considered the large spheres in these experiments to be the “limiting
reactant.” We determined the yield of our experiments by
identifying on each large sphere any vacant areas large
enough to bind a small sphere; each such defect was
considered to be a site of incomplete “reaction” of the small
spheres with the large sphere. In the large-field image shown
in Figure 3 b, there were 100 complete assemblies (including
those hidden by the inset), each with about 100 visible small
spheres. We found only six vacant sites in the entire image, for
a yield of greater than 99.9 %.
Each monolayer assembly had an overall net charge, as
shown by the response of an assembly to an applied electric
field (ca. 1 kV cm 1). The sign of the charge was the same as
that of the smaller (outer) beads in the assembly. Coulombic
interactions between nonpolarizable charged spheres can
explain the stability of these assemblies. As detailed in the
Supporting Information, simple electrostatic calculations
show that one of these structures can have a net electrostatic
charge and still have a negative total electrostatic energy. In
this case, the electrostatic attraction between the small
spheres and the large central sphere is greater than the
electrostatic repulsion between the small spheres, even when
the small spheres are close-packed on the surface.
Since the assemblies had a net charge, we attempted to use
them as components in subsequent steps of self-assembly.
Shaking and tapping, however, disrupted the assemblies.
Heating the assemblies to approximately 260 8C for 10–15 min
annealed the beads together, and made the assemblies more
robust mechanically. After annealing, we could use the
charged composite structures as components in a subsequent
assembly step (Figure 3 a) to yield multilayered microstructures (Figure 3 c). This process is reminiscent of the layer-bylayer electrostatic self-assembly of polyelectrolytes in solution.[5, 8]
The experiments shown schematically in Figure 4 a demonstrate that the self-assembly is due to electrostatic inter-
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Communications
Figure 4. a) A schematic representation of the experiment demonstrating that assembly involves electrostatic interactions (see text for
details). b) An optical micrograph of the assemblies resulting from a
mixture of 200-mm-diameter positively charged colorless spheres, 200mm-diameter negatively charged orange spheres, and 20-mm-diameter
positively charged colorless spheres. c) An optical micrograph (on the
same scale) of the assemblies resulting from a mixture of 200-mmdiameter positively charged colorless spheres, 200-mm-diameter negatively charged orange spheres, and 20-mm-diameter negatively charged
orange spheres.
Figure 3. a) A schematic representation of the procedure for multistep
electrostatic self-assembly of charged polystyrene microspheres.
b) Optical micrographs of the structures resulting from a combination
of 200-mm-diameter negatively charged orange spheres with 20-mmdiameter positively charged colorless spheres. c) Optical micrographs
of the structures resulting from the multistep self-assembly of 200-mmdiameter positively charged colorless spheres, 20-mm-diameter negatively charged orange spheres, and 70-mm-diameter positively charged
colorless spheres. Note the small orange spheres visible in the gaps
between the outermost spheres. See text for details. The images in (b)
and (c) correspond to the boxed structures b and c, respectively,
in (a).
actions. To a mixture of both positively charged (colorless)
and negatively charged (orange) 200-mm-diameter spheres we
added an excess of 20-mm-diameter positively charged (color-
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less) spheres. The small spheres coated only those large
spheres with the opposite charge, while the like-charged
spheres remained uncoated (Figure 4 b). To a similar mixture
of large spheres we added small negatively charged (orange)
spheres. Again, the small spheres coated only those large
spheres with the opposite charge (Figure 4 c). We obtained
the same results regardless of the order in which the three
batches of spheres were combined. As an additional test, we
gently agitated some of the self-assembled structures while
exposing them to ionized air from an anti-static gun (Zerostat) and found that the structures disassembled into individual microspheres. Finally, we performed a control experiment
in which we combined 200-mm and 20-mm unfunctionalized
polystyrene beads under the same conditions used for the
other assemblies. We observed almost no adhesion between
these unfunctionalized beads (Figure S2 in the Supporting
Information).
In conclusion, we used the ion-transfer model of contact
electrification to design microspheres that develop predictable electrostatic charges upon contact with other surfaces.
Oppositely charged microspheres self-assemble into uniform
spherical microstructures. By using contact electrification to
2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 206 –209
Angewandte
Chemie
produce charged components, we avoid the use of expensive
equipment such as a high-voltage power supply or an
electron-beam gun, and enable the use of large quantities of
material, or assembly over large areas. Our assemblies form
within seconds in greater than 99.9 % yield, and we can use
the assembled structures as components in multistep selfassembly. We assemble dry particles in air, and can assemble
particles as small as 5-mm in diameter. The use of dry selfassembly avoids the coagulation that can occur when a solid is
isolated from a liquid suspension, but it limits the minimum
particle size for this technique, as sub-1-mm dry powders tend
to form extended aggregates. We speculate that this process
could be used to encapsulate or coat microscale particles (e.g.,
for drug delivery), or to increase the accessible surface area of
a solid support in a controlled fashion. Although we have
demonstrated electrostatic charging and self-assembly only
with polystyrene microspheres, any dielectric material that
can be functionalized with covalently bound ions and mobile
counterions should demonstrate similar charging properties.
Received: July 20, 2006
Published online: November 30, 2006
[1] W. R. Harper, Contact and Frictional Electrification, Laplacian
Press, Morgan Hill, CA, 1998.
[2] J. Lowell, A. C. Rose-Innes, Adv. Phys. 1980, 29, 947.
[3] G. Decher, Science 1997, 277, 1232.
[4] P. Bertrand, A. Jonas, A. Laschewsky, R. Legras, Macromol.
Rapid Commun. 2000, 21, 319.
[5] F. Caruso, Colloid Chem. II 2003, 227, 145 – 168.
[6] C. F. J. Faul, M. Antonietti, Adv. Mater. 2003, 15, 673.
[7] M. Schonhoff, Curr. Opin. Colloid Interface Sci. 2003, 8, 86.
[8] P. T. Hammond, Adv. Mater. 2004, 16, 1271.
[9] D. M. Pai, B. E. Springett, Rev. Mod. Phys. 1993, 65, 163.
[10] B. A. Grzybowski, A. Winkleman, J. A. Wiles, Y. Brumer, G. M.
Whitesides, Nat. Mater. 2003, 2, 241.
[11] K. Mikihiko, S. Norio, D. Takehiro, F. Hiroshi, K. Takeshi, E.
Mitsuru, Proc. SPIE-Int. Soc. Opt. Eng. 2001, 4334, 263.
[12] B. Rezek, J. Stuchlik, J. Kocka, A. Stemmer, J. Non-Cryst. Solids
2005, 351, 3127.
[13] H. O. Jacobs, G. M. Whitesides, Science 2001, 291, 1763.
[14] C. R. Barry, J. Gu, H. O. Jacobs, Nano Lett. 2005, 5, 2078.
[15] A. F. Diaz, J. Adhes. 1998, 67, 111.
[16] A. F. Diaz, D. Fenzel-Alexander, Langmuir 1993, 9, 1009.
[17] A. F. Diaz, J. Guay, IBM J. Res. Dev. 1993, 37, 249.
[18] A. F. Diaz, D. Wollmann, D. Dreblow, Chem. Mater. 1991, 3, 997.
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Keywords: electrostatic interactions · polymers ·
polystyrene beads · self-assembly
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